The present invention relates to an image forming method as well as to an image forming apparatus, and more specifically to an image forming method as well as an image forming apparatus employed in the field of copiers and printers.
In recent years, widely employed as electrophotographic photoreceptors have been organic electrophotographic photoreceptors (hereinafter referred to as organic photoreceptors or simply photoreceptors) comprising organic photoconductive materials. As compared to other electrophotographic photoreceptors, organic photoreceptors exhibit the following advantages, that is, it is easy to develop materials which respond to various types of exposure light sources, ranging from visible light to infrared radiation; it is possible to select materials which do not result in environmental pollution; production cost is lower; and the like. The only defect is that the mechanical strength is not sufficient and during copying or printing a large number of sheets, the organic photoreceptor surface results in deterioration as well as abrasion.
As one method for improving mechanical abrasive wear properties of said organic photoreceptors, Japanese Patent Publication Open to Public Inspection No. 6-118681 discloses a hardenable silicone resin layer comprising colloidal silica which is used as the surface layer of a photoreceptor. However, the organic photoreceptor, in which said hardenable silicone resin layer comprising colloidal silica is used as the surface layer, exhibits insufficient electrophotographic properties, and during image formation employing many sheets, problems with a decrease in image density or background staining occur.
In order to overcome the drawbacks of said hardenable silicone resin layer comprising colloidal silica, the researchers involved with the present invention proposed to employ a charge transferable hardenable polysiloxane resin layer as the protective layer of an organic photoreceptor. The resultant organic photoreceptor, having said protective layer, exhibits improved abrasive wear resistance as well as improved electrophotographic properties, and during image formation of many sheets, neither decrease in image density nor background staining results.
In recent years, in image forming methods utilizing an electrophotographic system, a digital system image forming method, employing a laser beam image exposure, and the like, has been widely employed instead of the conventional image forming method, employing analogue system image exposure. In laser beam printers, which utilize the electrophotographic system, or copiers, facsimile machines, and the like, employing the digital system, images such as text, graphics, and the like, are formed on the photoreceptor utilizing a binary recording in which a laser beam is exposed or not exposed onto said photoreceptor. In other words, a dot image is formed for each pixel to form an image. In such image exposure utilizing a digital system, the widely employed method for processing images is that an image area such as text, graphics, and the like is exposed and the exposed area is visualized employing reversal development. When the image forming method, utilizing said digital system reversal development, is applied to the organic photoreceptor comprised of said charge transferable polysiloxane hardenable resin layer, images result in granular appearance and problems occur in which the reproduction of text is degraded or in a flat image (in which approximately uniform density dominates a constant area), the image disappears in the lower image portion. As a result, it has been found that the stabilizing effects for image formation of many sheets are not usefully utilized.
It is an object of the present invention to provide an image forming method and an image forming apparatus which can produce excellent electrophotographic images, employing an organic electrophotographic photoreceptor having a protective layer in which mechanical abrasive wear resistance is sufficiently improved, and more specifically to provide an image forming method and an image forming apparatus which result in excellent reproduction of halftones as well as text, employing an organic electrophotographic photoreceptor having a siloxane based resinous layer as the protective layer.
The inventors of the present invention have conducted various investigations to overcome the aforementioned problems. Discovered, as a result, was the relationship which affects image quality during image formation between the thickness of the protective layer of the organic electrophotographic photoreceptor having a siloxane based resin layer as the protective layer, and development conditions which develop a latent image in said organic electrophotographic photoreceptor, and thus the present invention has been realized.
Namely, it was discovered that the object of the present invention is achieved utilizing any of the embodiments described below.
1. In an image forming method in which a toner image is formed on an organic electrophotographic photoreceptor having a siloxane based resin layer as the protective layer, employing a developer material, an image forming method wherein distance Dsd (xcexcm) between said organic electrophotographic photoreceptor and the development sleeve, bearing said developer material, is related to the protective layer thickness t(xcexcm) of said organic electrophotographic photoreceptor so as to satisfy to Formula (1):
35t+400 less than Dsdxe2x80x83xe2x80x83(1) 
2. In an image forming method in which at least each of the processes of charging, image exposure and development is applied onto the surface of an organic electrophotographic photoreceptor having a siloxane based resin layer as the protective layer, an image forming method wherein said image exposure process is carried out employing a digital image exposure system, the distance Dsd (xcexcm) between said organic electrophotographic photoreceptor and the development sleeve, bearing a developer material employed in said development process, is related to the protective layer thickness t(xcexcm) of said organic electrophotographic photoreceptor, as well as to the difference xcex94V (V) between the voltage applied to said development sleeve and the electric potential in the exposed area of said organic electrophotographic photoreceptor so as to satisfy to Formula (2):
|(35t+400)xc3x97xcex94V|/500 less than Dsdxe2x80x83xe2x80x83(2) 
3. The image forming method described in 1. or 2. above, wherein said siloxane based resin layer has structural units having charge transportability as well as a bridge structure.
4. In an image forming apparatus which forms a toner image on an organic electrophotographic photoreceptor having a siloxane based resin layer as the protective layer, employing a developer material, an image forming apparatus wherein distance (Dsd) between said organic electrophotographic photoreceptor and the development sleeve, bearing said developer material, is related to the protective layer thickness (t) of said organic electrophotographic photoreceptor so as to conform to the aforementioned Formula (1).
5. In an image forming apparatus which possesses at least each of the means of charging, image exposure and development above the surface of an organic electrophotographic photoreceptor having a siloxane based resin layer as the protective layer, an image forming apparatus wherein said image exposure means is comprised of a digital image exposure system, and distance (Dsd) between said organic electrophotographic photoreceptor and the development sleeve, bearing a developer material employed in said development means is related to the protective layer thickness (t) of said organic electrophotographic photoreceptor as well as the difference (xcex94V) between the voltage applied to said development sleeve and the electric potential in the exposed area of said organic electrophotographic photoreceptor so as to conform to the aforementioned Formula (2).
6. The image forming apparatus described in 4. or 5. above, wherein said siloxane based resin layer has structural units having charge transportability as well as a bridge structure.